The present invention relates to a gaseous-discharge lamp, in particular for motor-vehicle headlamps.
Gaseous-discharge lamps or high-pressure discharge lamps are already a standard feature of motor-vehicle headlamps today, since they are much more efficient in terms of luminosity than conventional incandescent lamps, and because the spectral composition of their light is very similar to that of daylight. Depending on the ignition method used, these gaseous-discharge lamps require an ignition voltage between the electrodes of from 6 kV up to about 25 kV. This voltage initiates ionization in the gas filling. Small voltages of only about 50 V are still needed for the light to stay alight, i.e., to maintain the electric arc between the electrodes, since sufficient charge carriers are already present. However, producing high ignition voltages, particularly when working with HF-resonance voltage, places high demands on the electronic components being used and on the insulation of the lamp base, the lamp holder, and on the components that produce the high voltage (ignition inductor, ignition capacitor, etc.). Gaseous-discharge lamps of this kind, their use for motor-vehicle headlamps, and variants of ballast units for producing the ignition and maintaining voltage for such lamps are known, for example, from the German Published Patent Application No. 35 19 611 and from xe2x80x9cLamps and Lightingxe2x80x9d, second edition, S. T. Henderson and A. M. Marsden, p. 328 ff. Due to the problems caused by the high ignition voltage, one has generally striven to reduce the ignition voltage, while at the same time ensuring that a reliable ignition is maintained.
An advantage of the gaseous-discharge lamp of the present invention is that the ignition voltage is able to be substantially reduced, while a reliable ignition performance is maintained, with only relatively slight changes in the design of the lamp or of its electrodes being necessary. The resultant reduction in the requirements placed on the components contributes significantly to lowering costs when it comes to the electronic ballast unit and, also, when it comes to the gaseous-discharge lamp itself, since in this case, for example, the demands placed on the high voltage strength of the lamp base and of the components arranged therein are considerably diminished. The costs of the gaseous-discharge lamp are clearly reduced by integrating the ballast unit in the lamp base.
The ignition voltage can be lowered quite effectively by using at least one ignition electrode which can be configured separately from the main electrodes or integrally formed thereon.
According to another embodiment of the present invention, this ignition electrode can be designed as a separate third electrode having its own gas-tight electrode bushing traversing the burner vessel, the shorter ignition gap being formed toward one of the main electrodes.
In another advantageous embodiment, of the present invention the ignition electrode can be configured as a separate third electrode on the outside of the burner vessel and form, in turn, the ignition gap toward one of the main electrodes. This design requires only a very slight structural change to the conventional gaseous-discharge lamps; i.e., a later installation of this ignition electrode on conventional gaseous-discharge lamps is also possible, in particular on one of the tubular extensions which contain the electrode bushing for the main electrodes of the lamp. It is useful, in this context, for the ignition electrode to embrace one of the two main electrodes in an annular or semi-annular shape. If the tubular extension of the burner vessel has a constricted area, it is advantageous that the ignition electrode be advantageously arranged at this constricted area or extend into it, since this renders possible an especially short ignition gap and a corresponding clear reduction in the ignition voltage.
In all of the afore-mentioned embodiments, the ignition electrode can be designed either as a true third electrode or as a galvanic connection to one of the main electrodes, which enables the ignition section of the ballast unit to be operated completely separately from the remaining electronics. This means that only the ignition section of the ballast unit needs to be high-voltage proof, not, however, the majority of the components required for normal low-resistance operation. It is certainly possible, as well, for the ignition electrode to be electrically connected to the main electrode that does not play a role in forming the ignition gap, thus simplifying, altogether, the design and the voltage leads.
In another advantageous embodiment of the present invention, the at least one ignition electrode is configured inside the burner vessel, where it is better protected from external influences and where the connection to one of the main electrodes is able to be established easily and cost-effectively. This specific embodiment can be advantageously implemented by linking the ignition electrode to the one main electrode and having it extend up to one point situated near the other main electrode and underneath it in the operating state. It is useful in this context to design the ignition electrode as a rod- or wire-type side arm of the one main electrode, so that the ignition electrode can be manufactured together with the main electrode as a one-piece component, the unattached end of the ignition electrode leading, in particular, to the inner wall of the burner vessel, or, however, for the ignition electrode to be designed as a metallic coating on the inside of the burner vessel and, to facilitate connection to the one main electrode, to extend up to its electrode bushing, to automatically establish an electric connection. A metallization or metal-vapor deposition is to be carried out in this manner relatively inexpensively during the course of normal manufacturing of the lamp.
Starting from the electrode bushing, the metallic coating wraps at least partially around the main electrode and preferably extends for the most part up to the unattached end region of this main electrode, so that a creepage spark gap can form from there. A clearer reduction in the ignition voltage can be achieved by using a lamellar (i.e., strip-shaped element) or light-reflector type metallic coating that extends at least along the region of the arc gap up into the region of the other main electrode. The light-reflector type metallic coating preferably extends essentially over that half of the burner vessel""s combustion chamber which is the lower half in the working position and has the additional advantage of helping to assume the function of the screen that is otherwise required in a motor-vehicle headlamp for a lower beam, to adjust the mandatory light/dark cutoff and to protect oncoming traffic from glare. Given reflecting properties, the largest portion of the light that is otherwise lost is able to be used to illuminate the street, provided that the intended use is in a motor-vehicle headlamp.
In another advantageous embodiment, of the present invention each of the two main electrodes is linked to an ignition electrode, and formed between these as an ignition gap is a spark gap or creepage spark gap.
In this context, the ignition electrodes are designed in a first structural embodiment of the present invention as side arms of the main electrodes and extend up to the inner glass wall of the burner vessel, in particular to form a creepage spark gap. The ignition electrodes are configured here as rod- or wire-type arms or as pointed side shapes on the main electrodes, an especially high electric field being produced at the pointed ends, enabling a marked reduction in the ignition voltage. In the case of the rod- or wire-type arms, the ignition electrodes preferably extend obliquely toward one another up to the ignition gap and, in operation, are arranged underneath the main electrodes. This facilitates very short ignition gaps accompanied by a corresponding perceptible reduction in ignition voltage. Due to the thermal conditions in the combustion chamber, the electric arc formed following the ignition spark then travels automatically to the location between the main electrodes.
In an alternative structural embodiment, of the present invention the two ignition electrodes are conceived as metallic coatings, which extend up to the electrode bushings of the main electrodes to establish a connection with these electrodes. Here, as well, designs equivalent to those used for a single electrode formed by metallization are possible, the already described advantages also arising, in turn. When working with two ignition electrodes of this kind, even greater structural variations are possible, and creepage spark gaps can be simply formed as ignition gaps along the inner wall of the burner vessel.
Suitable, in particular, as a metallic coating is a tungsten metallic coating.
Another advantageous embodiments of the present invention lies in forming the main electrodes with a cross-sectional profile having an acute comer, in particular a triangular profile. Since the electrodes extend up to the inner glass wall at the electrode bushing, there is a very sudden rise in dielectricity at the glass/electrode separation point, resulting in high field strengths. This effect is reinforced by the acute corner, so that even in response to relatively low ignition voltages, a creeping discharge is produced at the glass wall. Here, in turn, as in the other exemplary embodiments, of the present invention the electric arc migrates upwards due to thermal effects and, eventually burns across the expanded cross-sectional area. This can also be reinforced in that the mutually facing surfaces of the main electrodes are inclined in opposition with respect to their longitudinal axes, the regions of the main electrodes that are closer to one another continuing as wider and those regions that are more distant from one another continuing as tapered.